1. Field of the Invention
The present invention relates to a process for producing a semiconductor device which has a protecting film comprising an impurity-containing glass.
2. Description of the Prior Art
A semiconductor device, for example a metal insulator semiconductor field effect transistor (MIS FET), is frequently provided with an impurity-containing glass film, especially a phosphosilicate glass (PSG) film, for insulating and protecting it. In producing such a semiconductor device, after the formation of the phosphosilicate glass film, the phosphosilicate glass film is heated to cause a controlled plastic flow of this glass film to smooth any sharp surface contour. This prevents a metal electrode or a conductive film from breaking when the electrode or conductive film is formed. Such a controlled plastic flow of a glass is described, for example, in U.S. Pat. No. 3,825,442. However, the heating treatment causes a very inconvenient problem, i.e. a very poor ohmic contact between an electrode and the semiconductor material surface. This inconvenient problem will now be explained referring to the attached FIG. 1 and FIG. 2.
FIG. 1 and FIG. 2 are partial cross-sectional views of a semiconductor device in intermediate stages of production in accordance with prior art techniques. In FIG. 1 and FIG. 2, reference numerals 1, 2, 3, 4 and 5 represent a silicon semiconductor substrate of n-type, a silicon dioxide film, a region having a diffused p-type impurity (a source or drain region), a phosphosilicate glass (PSG) film and a window for an electrode contact, respectively.
After the phosphosilicate glass film 4 is formed, the electrode-contact window 5 is opened in the phosphosilicate glass film 4 and the silicon dioxide film 2 by an ordinary photolithographic method (cf. FIG. 1). Then the phosphosilicate glass film 4 is softened by heating it, whereby the surface of the phosphosilicate glass film 4 becomes smooth and, especially, the edge portion of the glass film 4 defining the window 5 is provided with a gradual slope (cf. FIG. 2).
When such heating treatment is performed, phosphorus contained in the phosphosilicate glass film 4 is diffused out of the glass film 4, and this outdiffused phosphorus enters into the region 3 having the diffused p-type impurity. As a result, a region 6 (FIG. 2) having a diffused n-type impurity (i.e. phosphorus) is formed. Therefore, a PN junction is formed in the region 3, so that a good ohmic contact between a metal electrode and the surface of the region 3 cannot be obtained. Most persons skilled in the art would be of the opinion that prior to the heating treatment the window 5 should be opened in only the phosphosilicate glass film 4 while the silicon dioxide film 2 remains on the region 3. However, at present it is not known how effectively to selectively etch only the phosphosilicate glass film 4, so that the etching inevitably proceeds to the silicon dioxide film 2.